Method of making electrical resistance units



Oct. 23, 1934. G, E. MEGOW 1,973,163

METHOD OF MAKING ELECTRICAL RESISTANCE UNITS Filed Sept. 16, 1931 4Sheets-Sheet 1 WWW/1% 12 IN VE N TOR EFF/"L75 E Mgaw B) a ATTORNEY Oct.23, 1934. a. E/MEGOW METHOD OF MAKING ELECTRICAL RESISTANCE UNITS FiledSept. 16, 1931 4 Sheets-Sheet 2 C v fl & 7 K

ATTORNEY Oct. 23, 1934. G, E MEGOW 1,978,163

METHOD OF MAKING ELECTRICAL RESISTANCE UNI'I S I 1 Filed Sept. 16, 19314 Sheets-Sheet 3 INVENTOR 550F475 E Magaw er a.

A T TORNEY G. E. MEGOW METHOD OF MAKING ELECTRICAL RESISTANCE UNITS 4Sheets-Sheet 4 .Filed Sept. 16, 1931 v Ei 2$ 4 R mm &

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Patented Oct 23, 1934 UNITED STATES METHOD OF MAKING ELECTRICALRESISTANCE UNITS George E. Megow, South Milwaukee, Wis., as-

signor, by mesne assignments,

to Allen- Bradley Company, Milwaukee, Wis., a corpora/ tion of WisconsinApplication September 16, 1931, Serial No. 563,135

10 Claims.

- This invention relates to electrical resistance units and to a methodof and apparatus for mak- The present highly competitive nature of thisindustry has also necessitated every possible reduction in the cost ofthe units, and the exceptionally wide range of resistance valuesrequired, which runs from below one thousand ohms upwardly to over tenmillion ohms together with a demand for reduced sizeand-increasedmechanical strength, has made the production tot units having all thenecessary qualificationsextremely difficult.

To insure absolute stability of the unit, it is essential that theresistance material be protected against contact with moisture. The typeofunit which is best adapted to these needs is a ceramic unit of rodshape with the resistance material forming a lengthwise core, butdifliculty has been experienced with this type of unit in that .the coreof resistance materialshrinks away from the insulation materialsurrounding it. This results in an undesirable clearance between thecore and the insulation which invariably results in break- 0 age of thecore. Obviously such breakage of the core would change the resistancecharacteristics of the unit and in some instances would result in a deadopen'circuit. Another objection to even a slight clearance between theresistance material 5 forming the core and the insulating body is thatheat conduction from the core oi the insulating body is poor. a

With these and other objectionable features of the present ceramic typeof unit-in mind, this novel method and apparatus for making units ofthis character wherein a perfect bond is produced between the resistancecore and the insulating material surrounding it.

truding the resistance core and the insulation material surrounding it.

With the above and other objects in view which will appear asthedescription proceeds, the invention resides in the novelconstruction, combination and arrangement of parts substantially ashereinafter described and more particularly defined by the appendedclaims, it being understood that such changes in the precise eminventionhas as one of its objects to provide a' I Anotherobject of thisinvention isto provide simple and effective means for simultaneouslyexbodiment of the hereindisclosed invention may be made as come withinthe scope of the claims.

In the accompanying drawings, are illustrated several complete examplesof the physical embodiment of the invention constructed according to thebest modes so far devised for the practical application or theprinciples thereof; and in which: v

Figure l is a section view through a completed resistance unitconstructed in accordance with this invention;

Figure 2 is a view similar to Figure 1, but illustrating the unit in itscondition immediately after extrusion; v I Figure 3 is a cross sectionalview taken through Figure 2 on the plane of the line 3-3;

Figure 4 is a section through the die head oi. an extruding machineillustrating the manner in which the materials are extruded to form theunits;

, Figure 5 is a. view partly inside elevation and partly in section ofan extruding machine adapted for continuous operation;

Figure 6 is a view partly in side elevation and partly in section of amodified form of extruding machine wherein the resistance core and theinsulation body material are extruded by separately operable plungers;

Figure 7 is a view partly in side elevation and partly in section ofanother modified form of extrudi'ng machine;

Figure 8 is a side elevation of a unit constructed in accordance withthis invention, but having a helically shaped core;

Figure 9 is a cross sectional view taken through Figure 8 on the planeof the line 99;

Figure 10 is a section view through an extruding die head for formingthe units shown in Figure 8; and v Figures 1'1 and -12 are cross sectionviews through resistance units having cores of differently shaped crosssection,

a The completed unit constructedinaceordance with this invention isillustrated in Figure 1 and comprises a body lof insulating materialhaving a core 2 of resistance material extending longitudinallytherethrough. Both the body 1 and the resistance core 2 are formed 01porcelain forming clay or'other suitable ceramic material and theresistance core is given the desired degreapi'. conductivity by addingcarbon black. 5%)} At the ends 3 of the unit, the'insulating materialcontains carbon black which is deposited into its pores in the mannerbrought out in a copendapplication ofLaurnceE. Power, Serial No.

*- ance contacts to facilitate the connection of the unit in an electriccircuit.

.As illustrated in Figure 1 and as more at length defined in the saidcopending application, the conducting material at the ends 3, contactswith the extremities of the resistance core and metal caps 4 are.preferably pressed onto the ends 3 to enable wire leads, not shown, tobe soldered thereto so that the connection of the unit in an 1 electriccircuit is facilitated.

The manufacture of theunit as illustrated in Figure 1, comprisesgenerally the preparation of two batches of porcelain forming clay, onewith a predetermined percentage of carbon black and the other without;the simultaneous extrusion of both materials while still moist into longrods having the 'material containing carbon black forming a resistancecore within the non-conducting material, and cutting'the rods toappropriate lengths. After the rods are cut into pieces of the desiredlength the material is still fairly moist but is sufliciently dry tomaintain its shape. The

pieces are then dried at approximately room temperature (68 degrees F.)for three or four hours and then baked at a. temperature sufiicient toincrease their porosity. At the completion of this baking period, theends of the units are immersed in a carbonaceous liquid, which may be aphenol condensation product varnish dissolved in denatured alcohol. Thisliquid contains a relatively high percentage of fixed carbon whichpenetrates into the pores of the end portions of the unit.

. After having-been immersed, the pieces aresubjected to a temperatureat which the carbon is freed from its carrier and becomes fixed in theporous ends of the structure. The unit is also vitrified'during thislatter baking to a substantially glass-like hardness. At this, stage,the metal caps may be pressed onto the ends of the units.

The method by which the ends of the insula-,

tion body surrounding the resistance core are suring the desiredintimacy of contact between,

changed into a conductor to provide contacts, as stated forms the sbject matter. of the copending application, Seri No. 551,608, filed July18, 1931, whereas this invention deals with the simul-' taneousextrusion of the resistance material and the insulating material intothe rods from which the finished units are cut, and the preparation,

of the materials so as to obtain a closevunion between the resistancecore and the insulation body surrounding it. v I

Several methods have been developed for in- .the core and its insulatingbody. In each instance a difference in shrinkage, between the resistancecore and the insulation .material surrounding it is obtained, with theinsulating material shrinking more in volume than the resistance core sothat during the baking processes the core is securely gripped by theinsulating shell surrounding it.

One methodof obtaining the desired difference inshrinkage of thematerials consists in dividing the porcelain forming clay which has thedesired amount of carbon black mixed with it and of which the resistancecore is formed, into two batches of and 30% by weight. The largerportion iscalcined at approximately 2200 degrees F., a point on the lowrange of vitrification. This calcining reduces the shrinkage of thematerial. After being calcined, it is ground and mixed with theuncalcined material which is the batch of 30%. by weight. The porcelainforming clay of which the insulating body is-formed is not calcined, andhence after both materials are extruded in the manner illustrated inFigure 4, the greater shrinkage of the insulating material during bakingbinds the resistance core and-thus insures intimate contactbetweenthetwo materials. I All of the material of which the resistance core isformed is not calcined, so that the uncalcined portion provides a moistcarrier for the dry calcined portion. v i I I A'diflerence in shrinkagemay be also obtained by selecting a material for the resistance corewhich vitrifies at a higher temperature than the material of theinsulation body. To this end the material for the insulating bodycontains approxi- -mately 2% by Weight of sodium or potassium oxideswhereas the resistance core is formed of porcelain forming clay towhich; is added carbon black in the percentage required for theresistance desired. During the firing processes, thejn'sulating bodyvitrifies before the core and thus shrinks of carbon black, and it hasbeen found that this material in different degrees of subdivisionhasdifi'erent rates of shrinkage. Hence, during Tthe firing, the insulatingbody being in a finer state of sub-division, shrinks to a greaterdegree.

Intimacy of contact between the core and the insulating materialsurrounding it,'.also results from the specificconstruction of theextruding die. The shape of the extruding'die, see Figure 4, is suchthat the materials forced therefrom are compressed to a substantialdegree so that any voids or clearance between the core and the insulating shell is impossible.

In some instances it has been found desirable to have the resistancematerial in a moist or semi-fluid state during the extrusion.Compression of the materials at the point of extrusion is thusimpossible. In this case the extruded rods are allowed to dry for apredetermined period .of time and with the insulating material still ina plastic state, the rods are compressed in a compression die or anyother suitable mechanism.

The simultaneous extrusion of the materials may be' obtained in-severaldifferent ways, and in Figures 5, 6 and 7 three types of extrudingmachines which are particularly well adapted for this purpose, areillustrated andreference is now specifically directed to these figuresof the drawings.

Referring to the machine illustrated in Figure 5, which is the preferredembodiment in that it permits continuous extrusion, the numeral 5represents a cylinder supported by afoot 6. Inthe bore 7 of the cylinderisa tubular conveyer screw 8 and a material receivinghopper 9communicates with the upper portion of the cylinder 5 to receive thematerial, of which the insulating shell of the units is to be formed.

The forward end 10 of the cylinder is counterbored and internallythreaded, as at 11, to mount a member 12. This member 12 has a centralbore 13 which is" straight for the major portion of its length andflares outwardly, at 14, at itsinner end to substantiallythe diameter ofthe cylinder bore 7. The bore 13. with its flared inner end thus forms acontinuation of-the cylinder bore '7 and the screw conveyer 8 which isrotat able in the bore 7, is arranged to force the material, depositedin the hopper 9, outwardly through the bore 13.

Mounted at the front end of the conveyer scre 8 is a nozzle-like member15 whose outer surface is shaped to lie parallel with the adjacentportion of the flared inner end 14 of thebore 13. The space between themember 15 and the flared bore 14 permits the passage of material broughtforward by the conveyer screw 8 into the opening 13.

The outer end of the member 15 extends to the juncture of the flaredbore 14 with the straight bore 13 and has a small central bore 16extending therethrough. The inner end of the bore 16 is taperinglyenlarged, as at 17, to the diameter of av bore 18 extending axiallythrough the screw conveyer 8.

-Mounted within the bore 18 is a second spiral screw conveyer 19 whichextends outwardly beyond theinner end of the screw 8 into a bore 20which is a continuation of the bore 18 and is formed in a supportingmember 21. The upper end of the supporting member 21 has a hopper 22 forthe reception of the material which' is to form the core of the finishedunits.

Upon rotation of bothscrew conveyers 8.and 19, the respective materialsconveyed .thereby'are forced forwardly to the die head which comprisesthe members 12 and 15, and is forcibly extruded therethrough as bestillustrated in Figure 4, to form a continuous rod having a core ofresistance material surrounded by a body of insulating material. Anysuitable support may be provided for the rod as it issues from the diehead, and after the desiredlength has been extruded it is :cut

off and removed.

The screw conveyer 8 is held against longitudinal movement in the bore'7 by a flange 23 formed on its inner end which engages the adjacent endof the cylinder 5, and by having its extreme inner end abutting thesupporting member 21. The internal screw conveyer 19 is held againstlongitudinal movement in its bore by a screw 24 threaded in thesupporting member 21 and projecting into the bore 20 to engage its innerend in an annular groove 25 formed in the adjacent portion of the screwshaft.

The screws Band 19 are rotated in opposite directioris and driving forceis imparted thereto through gears 26 and 27 fixed to the screws 8 and19, respectively, the gear 26 for the outer screw being confined betweenthe flange 23 and the adjacent end of the supporting member 21 and thegear 2'7 being fixed to the end of the screw conveyer 19 outwardly ofthe supporting member 21. By forcing the material through the die headby means of screw conveyers, the materials are compacted and air pocketsare prevented inasmuch as the solids are carried forward by the actionof the screws and air and other gases have ready egress through thehoppers 9 and 22.

The importa' t advantage of this type of extruding machine, however, isthat it enables continuous extrusion rand the die head, which consistsof the members 12 and 15 is readily detachable so that any one machinemay be readily adapted to the extrusion of units of different diameters,it being necessary only to apply the proper die head. In Figure-6 isillustrated a modified form of extruding machine. In this construction,an upright cylinder 30 is mounted'on a bed plate 31 upon which ahydraulic cylinder 32 is also mounted at a distance from the uprightcylinder 30. Adjacent the bottom of the cylinder 30 plug of a member 34which corresponds to the member 12 in the construction illustrated inFigure 5 and forms .part of the extruding die head. As in the'structureshown in Figure 5, the member 34 has a longitudinal opening 35 whoseinner end flaresoutwardly, as at 36, to communicate with the interior'of the cylinder 30. l

.Mounted in axial alignment with the member 34 is a horizontal cylinder3'? of relatively small diameter and which extends transversely acrossthe bottom of the cylinder 30 to mount a nozzle member 38 whichcorresponds to the member 14 in the structure of Figure 5, and extendsinto the opening 35 in the member 34, and with the member 34 forms thecomplete extruding die head.

The transverse cylinder 3'7 is fixed in a nipple 39 threaded in anopening 40 formed in the wall of the cylinder 30 opposite the threadedopening 33. Slidably received within the cylinder 37 is the ram 45 of apiston 46 operating in the hydraulic cylinder 32.

A packing gland 4'7 provides a tight seal between the head 43 and theplunger 41. The hy- .draulic cylinder 32 is of conventional constructionand has means, not shown, for admitting a suitable fluid under pressureon either side of the piston 46 so as to enable the piston 46 andconsequently the plunger 41 actuated thereby, to be moved'in eitherdirection. Operating in the cylinder 30 in a manner similar to theplunger 41 is a piston or plunger 48 whose outer end passes through ahead 49 similar to the head 43 to be operated by a hydraulic ram, notshown, in the'manner in which the plunger 41 is operated. a

To load the cylinder 3'? with resfstance material to be extruded as thecore of the units, the connection 44 between the plunger 41 and the ram45 is disconnected and the entire cylinder 30 with its structureassembled thereon is swung about the axis of the cylinder 30 to enablethe plunger 41 to be removed. The rotation of the cylinder 30 about itsaxis is made possible by the rotatable mounting 50. Any

suitable means, not shown, may be provided for moving the drivingmechanism of the plunger 48 out of line therewith to enable completeremoval of the plunger from the cylinder 30 and the depositing of thematerial to form the insulating shell, therein.

In th s form. of the inventiommeans are pro vided for positivelywithdrawing any air in the cylinders in which the plastic material isreceived and to this end, both the head 43 and the head 49 have ports 51leading to the interior of the cylinders 3'7 and 30., respectively. andthe outer ends of these ports havetubes 52 leading therefrom forconnection with anysuitable mechanism for producing a vacuum. The means-.for creating the vacuum within the cylinders through the tubes 52 andthe ports 51, is set in operation before the plungers begin theircomextrusion of materials is from cylinders having plungers operatingtherein and actuated from hydraulic rams. This structure differs,however, from that shown in Figure 6 in that both cylinders are axiallyaligned and that both their plungers are actuated from a single source.

The supporting structure of this form of extruding machine consists oftwo side plates 53 between one end of which a hydraulic'cylinder 54. ismounted. Positioned between the other or outer ends of the side plates53 are two axially aligned 'telescoped cylinders 55 and 56. The

outer ends of both cylinders are fixed to a head 57, the outercylinder55 by being threaded, as at 58, in a counterbore formed in the head, andthe inner cylinder 56 by being threaded, as at 59, in a hub 60 spacedfrom the inner walls of a bore 61 in the .head, by spider arms 62.

The space in the bore 61 between the arms 62 and the hub 60 'iscommunicated'with the interior of the outer cylinder 55 and with the'longitudinal bore 63 in a nozzle member 64 forming part of the die head.This'member 64 is similar to the member 12 of the structure shown inFigure 5, and to the member 34 of the structure shown in Figure 6,' andlikewise has. the inner end of its bore fiaringly enlarged, as at 65. Aninner nozzle member 66 which cooperates with the member 64 to, completethe die head, is secured in the hub 60 outwardly of the inner cylinder56 and has its tapered bore forming a continuation of the innercylinder.

The cylinders 55and 56 are supported from the side plates 53 by ivotpins or screws 6'7, secured in the side plates and having their endsprojected into diametrically opposite openings formed in the outer wallof tlre cylinder 55. The cylin- -ders 55 and 56 are thus pivotallymounted for swinging movement about'the axis of the pins 6'7 for apurpose to be later described, and to hold the cylinders in proper axialalignment with the hydraulic cylinder 54', a second pair of removablescrews or pins 68 is detachably secured in the plates 53 to project intoopenings formed in the head 57.

Sliding in the bore of' the cylinder 56 is a plunger 69 whose stem '70is connected, as at 7'1, with a member 72 to which a sleeve '73 is alsosecured as at 74. The opposite end of the sleeve 73 is secured to a ring'75 .whichfills the space between the outerwall of the inner cylinderand the bore of the outer cylinder 55 to form a piston for the cylinder55. This sleeve 73 passes through an opening 76 in ahead 77 detachablysecured to the cylinder 55 and a suitable packing gland 78 is providedto afford a tight seal between the sleeve 73 and 'the bore 76 in thehead. 1

The member 72'has a detachable connection 79 with the ram 80 of thehydraulic cylinder 54 and as in the construction shown in Figure 6," thecylinder 54 is provided-with means, not shown, for conducting fluidunder. pressure to either side of the piston 81 operating therein so asto enable the ram 80 and consequently the plungers 69 and 73 to be movedin either direction:

It is observed that the bore of both inner and outercylinders'isrelieved, as at 82 and 83, respectively, at the ends of the cylindersadjacent the head 77 so that when the plungers 69 and 73 are fullywithdrawn an air space is afforded past the plungers. The space past theouter plunger 83 enables communication between the interior of ing avacuum, not shown, and by reason of the communication of the bore in thecylinder 56 the material to be extruded may be effectively eliminated.The open end of the bore 63 in the member 66 is preferably closed bysome suitable plug, not shown, during the process of withdrawing the airfrom the material.

The pivotal mounting afforded by the pins 67 enables the cylinders to beswung out of alignment with the ram 80 upon removal of the connection'79 to enable the head 7'7 and the plungers 69 and 73 to be removed fromthe cylinders to facilitate loading of, both cylinders with theirrespective materials. In.this type of machine it is essential that theareas of the plunger heads be proportioned correctly so that thematerials are extruded at the proper rates of volume.

Again referring to the extruding machine disclosed in Figure 5, itfisobserved that the opposite rotation of the screw conveyers which forcethe materials forwardly to be extruded, causes the materials to rotatewith respect to each other.

This relative, rotation on the part of the materials as they passthrough the extruding die head may be-utilize'd to produce a unit inwhich the resistance core is of helical shape'so as to increaseitslength without increasing the overall length of the unit.

A unit having a helically shaped resistance core is illustrated inFigures 8 and 9, and in FlgurelO is illustrated the manner in which therelative rotation of the materials as they pass through the extrudingdie head is utilized to form the helical core. As clearly shown inFigure 10, the member 15 which is carried at the outer end of the screwconveyer 8 is replaced by a member 15' which 'is in all respects similarto the member 15 except that itsouter. end and its opening 16 is oilcenter.'

Thus as the screw conveyers rotate the outer end of the member 15' andthe bore 16' rotate in a circle about the axis of the bore 13.

If great contact area is desired between the core. and the insulationmaterial surroundingit, the opening in the inner member of the ex-'truding die head may be of irregular shape and in Figures 11 and 12, twodifferent cross sectionalshapes of cores are illustrated, each of whichincreases the contact area between the core and the insulation material.

From the foregoing description taken in connection with the accompanyingdrawings, it will be readily apparent to those skilled in the art towhich an invention of the character described appertains, that theherein described method of forming electrical resistance units enablesthe production of a ceramic type unit in which,a core of resistancematerial is embedded in a rod-like body of insulating material in apractical and economical manner, and thatby-reason of the difference inshrinkage between athe materials forming the core and the insulationsurrounding the core, a perfect bond is secured between the core and itsenclosure.

What I claim as my invention is:

1. The method of making an electrical resistor element which includesembedding a vitrifiable material containing a conductor and a portion ofwhich material has been calcined to decrease the "the whole and causethe encasing envelope to com press the inner core.

2. The method of making an electrical resistor element which includesembedding a vitrifiable material containing a conductor into an insulating material which vitrifies at a lower temperature than the firstmaterial, and firing the formed materials at a temperature to vitrifythe first material.

3. The method of making an electrical resistor element which includesembedding a vitrifiable material containing a conductor into avitrifiable material which is in a state of greater sub-division thanthe first material and firing the formed materials to a stateofvitrification.

4. The hereindescribed method of making an electrical resistor elementwhich comprises simultaneously extruding an insulating material and aresistance-material with the resistance material forming a core embeddedin the insulating material, the insulating material having a higherdegree of shrinkage upon subjection to heat than the resistancematerial, and in vitrifying the extruded materials to cause theinsulating material to compress the core of resistance material.

5. The hereindescribed methodof forming an electrical resistance elementwhich comprises preparing two batches of porcelain forming clay, onewith a percentage of conducting material to form a resistance material,and the other without, in treating one batch of materials so that theresistance material has a lesser degree of shrinkage truded materials toa vitrifying temperature.

upon subjection to heat than the other material, in simultaneouslyextruding the materials with the resistance material forming a corewithin the insulating material, and in subjecting the exwhereby thegreater degree of shrinkage of the insulating material insures intimatecontact between the resistance core and the insulating material.

6. The method of making an electrical resistance unit which includessimultaneously extruding an insulating material and a resistancematerial through eccentricall: disposed discharge ports one within theother to form a body of insulating material having a core of resistancematerial, and revolving one of the discharge ports about the axis of theother so that the resistance core is given a helical shape.

'7. The hereindescribed method of making an electrical resistor elementwhich comprises extruding an insulating material and a resistancematerial with the resistance material forming a core embedded in theinsulating material, said' materials having diflerent degrees ofshrinkage upon subjection to heat with the insulating-material having ahigher degree of shrinkage, and in vitrifying the extruded materials tocause the insulating material to compress the core of resistancematerial.

8. The method of making an electrical resistor element which comprises,treating a mixture of vitrifiable material and conducting material torender the same plastic, calcining a portion of said treated mixture tominimize its possible shrinkage upon subjection to heat; remixing thecalcined portion with the uncalcined portion of said mixture to providea core forming batch of material, preparing another batch of plasticvitrifiable insulating material which contains no conducting material,embedding a quantity of materialfrom said first batch in a shell formedof material from said second batch to form a body of the desired sizeand shape, and firing the formed body to a state of vitrification, thelesser possible shrinkage of the core causing the shell to compress thecore during vitrification.

9. The hereindescribed method of making an electrical resistor elementwhich comprises mix ing a quantity of a plastic vitrifiable materialwith a predetermined quantity of conducting material to provide a coreforming batch, mixing a fluxing agent with a quantity of the sameplastic vitrifiable material to provide a shell forming batch which, byreason of its content of fiuxing agent, vitrifies and contracts at alower temperature than the core forming batch, extruding said materialssimultaneously to form a body having a cbre composed of material fromthe first mentioned batch and an insulating shell composed of materialfrom the second mentioned batch, and firing the formed. body to vitrifythe same throughout its entire internal structure so that the insulatingshell contracting in advance of any contraction of the core shrinks ontoand compresses the core.

10. The hereindescribed method of making an electrical resistor element,which comprises preparing two batches of plastic vitrifiable material,one with a conducting material therein, and the other without, reducingthe material of the second designated batch to a finer state ofsub-division than that of the other batch, embedding a quantity ofmaterial from said first designated batch in a quantity of material fromsaid second designated batch to form a body having a resistance core andan insulating shell, and firing said formed body to a state ofvitrification during which the insulating shell shrinks onto the core.

' GEORGE E. MEGOW.

